1. Field of the Invention
The present invention relates to an image reading apparatus for scanning a document and reading image data of the document.
2. Description of the Related Art
An apparatus for processing image data such as a facsimile apparatus or a digital copying machine is provided with an image reading apparatus for scanning a document, reading image data of the document, and outputting an image signal. The image reading apparatus places the document on a document table, scans the document by means of a scanner in which a photosensor, etc. are built in, and reads image data of the document. In the image reading apparatus used in a facsimile apparatus, while a document is transferred by a document transfer device, image data is read from the document. In this manner, the image data is obtained while the document and the document reader for reading this document are moved relative to each other.
A pulse motor is employed for the scanner and document transfer device for moving the document and the document reader. In accordance with the rotation of the pulse motor, the document is scanned. In this type of the image reading apparatus, in order to change the document reading magnification, the scan density in a sub-scanning direction is changed. To change the scan density, the rotation speed of the pulse motor is changed, and accordingly the moving speed of the scanner and the transfer speed of the document transfer device are changed. For this purpose, the frequency of an excitation pulse to be supplied to the pulse motor is changed.
The pulse motor has a natural frequency. When the natural frequency of the motor and the frequency of the excitation pulse coincide with each other, there is a concern that a resonance or hunching occurs in the pulse motor and an obtained image may blur. It is thus necessary that the scan density at the time of reading the image data of the document be set in a range excluding a resonance range corresponding to the natural frequency of the pulse motor. Consequently, it is not allowed to freely set the scan density. Under the circumstances, there is an increasing expectancy for the advent of an image reading apparatus wherein a read image is stable.